This invention relates to aqueous inks for ink jet printers, and, more particularly, to aqueous pigmented inks containing cosolvents.
Ink jet printing is a non-impact method that, in response to a digital signal, deposits droplets of ink on a substrate such as paper or transparent film. Ink jet printers, especially thermal ink jet printers, have found broad application with personal computers in the office and home.
Thermal ink jet printers use a battery of nozzles, each containing a resistor element, to fire ink droplets toward the print media. Nozzle openings are typically about 40 to 60 micrometers in diameter. Thermal ink jet printers require inks that do not plug these small openings.
Therefore, a critical requirement for an ink jet ink is the ability to remain in the fluid condition in a nozzle opening on exposure to air, the so-called "decap" conditions. This ability allows a nozzle to function over a period of non-use or during operation of infrequently utilized nozzles.
Pluggage can be caused by evaporation of organic solvent or water from the surface of the nozzle. In pigment-based inks this evaporation can cause precipitation of the dispersion, flocculation of the pigment dispersion, and precipitation of solid additives.
Initial evaporation generally causes an increase in viscosity that affects the ability of the nozzle to fire a drop of ink since ink jet nozzles are designed to operate within specific viscosity ranges. The inception of pluggage may cause distortion of the image or alphanumeric character. This may appear as a drop of ink that is displaced from its intended position. Sometimes two drops will be formed equally spaced from the intended target position. In other instances, numerous small satellite drops are produced. On some occasions the drop may even reach its intended position but at a lower drop volume, producing a lower optical density image. Ultimately the plugged nozzle will fail to fire and no image will be generated.
Therefore, a critical property for an ink jet ink is the "decap time," which is the length of time over which an ink remains fluid in a nozzle opening when exposed to air and therefore capable of firing a drop of ink at its intended target. "Decap" means the nozzle is uncovered or "decapped." A typical "decap" test is run by firing all nozzles successively 100 times at several programmed incremental increasing time intervals. Each ink is given a time score for the first, fifth and thirty-second drop firing. This time interval is the longest interval that the nozzle fires a specified drop without drop displacement or loss of density. The longer the time rating, the more desirable the ink.
A commonly used scheme to cure pluggage is to clear the plug by firing the nozzle in a non-image mode, i.e., into a collection receptacle. While this solution is the most effective remedy, it requires that the ink form a "soft" or noncohesive plug. To make this non-image pluggage clearance effective, the surface plug must be mechanically or cohesively weak.
The ability of a plug to be removable by non-image firing is measured by the fifth and thirty-second drop decap time values. It is highly desirable to obtain a long time interval for the fifth drop because this means the plug is easily removed with only four non-image firings. An ink with long decap values for both the fifth and thirty-second drop indicates that a soft plug forms and is readily cleared.
Another important requirement for ink jet inks where the colorant is a pigment is that the pigment dispersion remains stable throughout the life of the ink jet cartridge. Some cosolvents that inhibit plug formation or promote only soft plugs generally cause destabilization of pigment dispersions and therefore cannot be used in pigmented inks.
U.S. Pat. No. 5,205,861, to Matrick, teaches an ink jet ink comprising an aqueous carrier medium, a colorant, and a nitrogen heterocyclic diol cosolvent. If the colorant is a pigment, the reference teaches that the pigment is contained in a polymeric dispersant. It is believed that these pigment-containing inks are in use in the HP 1200C ink jet printer. The reference does not teach an ink composition containing low molecular weight polyethylene glycols and/or sulfolane.
The print zone of a "heat and delay" printer, such as the HP 1200C, represents an extreme challenge for thermal ink jet devices. The front face is bathed in hot, dry air whenever it is uncapped. Use of the nitrogen heterocyclic diol cosolvent allows for good decap times even in a "heat and delay" printer. However, when the ink is force dried on a glass slide, the wet edge contracts towards the center, leaving rings of dried ink and a highly fluid center. The center remains highly fluid for several hours. Therefore, the ink is susceptible to print smudges.
References such as U.S. Pat. No. 5,180,425, to Matrick et al., U.S. Pat. No. 5,356,464, to Hickman et al., and U.S. Pat. No. 5,100,469, to Pontes et al., teach the use of both sulfolane and polyethylene glycols as cosolvents in ink jet ink. However, these references do not teach the selection of low molecular weight polyethylene glycols or its combination with sulfolane to form inks that inhibit the formation of plugs that are hard to remove.
In addition, although U.S. Pat. No. 4,585,484, to Haruta et al., and U.S. Pat. No. 4,627,875, to Kobayashi et al., teach dye-based recording liquids containing both polyethylene glycol and sulfolane, they do not teach pigmented ink jet inks.
A need exists for cosolvents that function as pluggage "softeners" or inhibitors without destabilizing pigment-based inks.